Everything works except the /node/[nr]/edit page of only one content type page. And it's not for every page of this specific content type. Only one.
So let's say I have a content type "product" and created 5 nodes with that content type. Editing 4 of them works but for one the error 500
Premature end of script headers: index.php occurs.
I tried increasing the memory limit in the php.ini all the way up to
memory_limit = 1024M (and restarted apache after that) but this changes nothing. Btw I'm using Acquia DevDesktop and the PHP version is 7.0.15. I also tried downgrading the PHP version (and there also increased the memory limit) but this won't do anything either.
Please help me, I'm incredibly thankful for every answer I can get!